April 25, 2012 -- There's no threat from the single California dairy cow yesterday reported to have mad cow disease. But what about other cows?

The USDA says the detection of the California cow and its removal from the food chain shows that our food safety program is working. But critics say we've just been lucky.

Since 2006, the USDA has been testing only 40,000 cattle a year for mad cow disease (bovine spongiform encephalopathy, or BSE). But each year we slaughter 35 million cattle. That means 99.9% of cattle are not tested. According to a 2006 USDA estimate, about one in a million U.S. cattle carries the disease.

"Our test program is so small, we are in no position to know if more of it is out there," Jean Halloran, director of food policy initiatives at Consumers Union, the policy arm of Consumer Reports, tells WebMD. "Even if BSE occurs spontaneously at a rate of one in a million, we would have three cases a year. And we are leaving a door open for this to circulate and amplify."

Only USDA Can Test for Mad Cow Disease

In 2006, a firm called Creekstone Farms set up a testing facility and asked the USDA for the test. Creekstone wanted to sell its Angus beef to Japan, which won't accept untested cattle over 20 months old. But the USDA refused to license the test to Creekstone. In a split decision, an appeals court upheld the USDA's position.

The USDA's position is that since the rapid BSE test can't detect early BSE infections, the test can't be used to certify -- or market -- beef as BSE-free. But Consumer Reports argues that letting companies test would at least increase the number of tests at industry expense.

"Japan tests every animal at slaughter. They know that once in a while one will slip through. But that's better than letting them all slip through," Halloran says.

In defense of the USDA, BSE testing likely would not detect the disease in recently infected animals. Why?

BSE is caused by a buildup of abnormal proteins (called prions) in animals' brains. It takes two to eight years -- five on average -- for an animal to develop BSE symptoms. The rapid test can't detect BSE until an animal is on the verge of symptoms. Most U.S. food cattle -- but by no means all -- are slaughtered before they are 2 years old.

Mad Cows From Beef Feed?

There's another issue. The reason why mad cow disease became a problem is that ground-up cattle were being put in cattle feed. The FDA banned this practice in 1997. The USDA points to this ban as a major factor in the safety of U.S. beef.

But chickens can be given feed containing pellets of ground-up beef.

"Chickens are messy eaters. They knock a lot of this pelletized feed on the floor -- then they sweep up the chicken litter and feed that back to the cows," Halloran says.

It takes years for a person infected with BSE to develop the fatal human disease called variant Creutzfeldt-Jakob disease or vCJD. Are there cases out there waiting to be diagnosed?

The USDA thinks not. The agency points to its program of prohibiting meat renderers from using any brain or spinal-cord tissues in any food product as well as to its testing program.

"The safety of our food is addressed through our interlocking safeguards and removal of any type of material that contains the BSE agent in the U.S.," USDA Chief Veterinary Officer John Clifford said yesterday in a video news release. "Our livestock population is one of the healthiest in the world. Consumers should be confident in our food supply."

Halloran says that consumers worried about BSE in meat can reduce their risk by avoiding cuts of beef most likely to transmit the disease. Organ meats, she says, are the least safe, followed by bone and by ground beef products such as sausage and hamburger.

But there may be a better way to stay safe: Buy 100% grass-fed or certified organic beef.

Clifford maintains there's no cause for concern.

"Our food safety inspection service oversees production to make sure materials like brain and spinal cord are removed appropriately and not allowed to enter the food supply," he says.